1. Field of the Invention
The present invention relates to a high temperature superconductor, in particular to a high temperature superconductor known as coated conductor.
2. Description of Related Art
Coated conductors, which are also referred to as “second generation superconductors” typically have a long length as required, for example, in the production of wires and cables.
They are composed of a substrate onto which is applied a multilayer structure with a high temperature (hts) superconductor layer and at least one buffer layer between the substrate and the high temperature superconductor layer. Typically, the buffer layer(s) are made of ceramic material having high resistance. The buffer layer(s) serve to compensate for the various different properties of the materials used. For example, buffer layers can be provided for preventing poisoning of the superconductor layer by constitutional elements of the substrate which might diffuse towards the superconductor layer.
Preferably, the hts layer is covered by a protection layer made of a noble metal or noble metal alloy.
In general, high temperature superconductors, such as coated conductors, are promising candidates for a plurality of applications such as high and medium voltage power transmission cables, wire applications, fault current limiters, transformers, magnets (for example, for magnetic resonance imaging) and synchronous motors with superconducting windings.
In the superconducting state the resistance of a high temperature superconductor material is practically zero. However, all superconductor materials can loose their superconductor properties by exceeding one or more of the critical temperature (Tc), the critical magnetic field (Hc) or the critical current (Ic). These factors are given for a specific superconductor material.
In practical, the superconductor material of a superconductor component such as a coated conductor is not completely homogenous throughout the whole component so that the above properties such as the critical current can be different in different regions of the superconductor layer. Consequently, in case of fault such as a large surge current, some regions of the superconductor layer may loose their superconductor properties and become resistive whereas other regions still remain superconducting. Due to the still superconducting regions high current flows through the superconductor layer which leads to a temperature increase in the already resistive regions and may cause burn out or thermal run-away.
For avoiding break down and damage in case of fault all conductive layers within the layer structure, such as the hts layer and the metal protection layer, must be electrically connected to the substrate. Since the substrate and the hts layer with or without metal protection layer, are electrically separated by the ceramic buffer layers an electrical connection must be provided which bridges the insulating buffer layers.
Typically, a coated conductor has a tape-like shape with plane faces wherein the layer structure is applied onto one of the plane faces.
Recently, also coated conductors with circular cross-section have been described, wherein the substrate forms a core which is covered by the layer structure. The core may be hollow, such as a tube, or may be solid such as a rod. For example, such “round coated conductors” and methods for production thereof are disclosed in US 2008/0119365 A1 and EP 1 916 720 A1 which are incorporated herein by reference.
Another example is disclosed in US 2007/0232500 A1.
In flat coated conductor tapes electrical connection of the metal protection layer and the superconductor layer with the substrate can be accomplished by simply providing a metal connection along at least the longitudinal edges of the tape with the metal connection extending from the substrate to the metal protection layer and forming a conductive path.
However, this solution is not applicable in a coated conductor wherein the layer structure surrounds the whole periphery of the substrate as in a coated conductor with circular cross-section with the layer structure surrounding the core.
According to US 2007/0232500 A1 this problem is solved by bending a pre-fabricated tape composed of the layer structure along its longitudinal axis around the core. As such the opposite longitudinal edges of the tape composed of the layer structure ubut face to face forming a longitudinal slot therebetween. The longitudinal slot extends from the outer surface of the layer structure to the surface of the core. For forming an electrical connection between the metal protection layer and the hts layer underneath the metal protection layer and the core the slot is filled with a metal.
However, according to this solution the position of the electrical connection is pre-determined and cannot be freely selected. Further, this solution is restricted to the case that a pre-fabricated tape-like layer structure is bent around a core, so that a longitudinal slot is formed.
U.S. Pat. No. 6,552,415 B1 and DE 40 04 908 A1 relate to a method for providing a conductive path in a coated conductor composed of a substrate, buffer layer(s) and a high temperature superconductor layer by providing through-holes which extends through the high temperature superconductor layer and the buffer layer(s) to the surface of the substrate. Then, a metal protection layer is deposited onto the high temperature superconductor layer, wherein the metal fills up the through-holes thereby forming a conductive connection with the substrate.
According to DE 40 04 908 A1 this method is also applied to a cylindrical coated conductor.
However, there is a disadvantage, that through-holes must be provided through the high temperature superconductor layer with the risk that the high temperature superconductor layer is impaired.